Rubber-reinforcing cords comprised of a core comprised of glass fibers or organic fibers made of rayon, nylon, a polyester or the like, and a rubber coating on the surface of the core, containing resorcin-formaldehyde which has high affinity to the matrix rubber, are embedded in rubber products such as timing belts and tires. It is known that in the case that such a rubber product is placed under a high-temperature high-humidity environment, or is used in oil, the rubber coating rapidly deteriorates, resulting in a marked drop in the strength of the rubber product. Moreover, in the case that such a rubber product is used under a low-temperature environment, the matrix rubber and the rubber coating will break upon being subjected to impact due to being brittle, and hence there will again be a marked drop in the strength. For example, timing belts used in a cold region are used under a harsh environment, for example being subjected to impact while being still brittle upon engine startup, and then subsequently being subjected to a high temperature due to waste heat from the engine. In particular, in recent years there has been a trend toward further increasing the density in engine compartments, and hence timing belts are used under yet higher temperature environments.
The rubber coating is formed by applying a solution (hereinafter referred to as a “fiber treatment agent”) containing an essential rubber component comprised of a resorcin-formaldehyde water-soluble condensate (hereinafter referred to as an “RF condensate”), and if appropriate another rubber component comprised of a hydrogenated butadiene-acrylonitrile copolymer latex, a vinylpyridine-styrene-butadiene copolymer latex or the like, and other components such as age resistors, emulsifiers and/or surfactants onto fibers that form the core, and drying and thus curing. Rubber-reinforcing cords obtained by impregnating such a fiber treatment agent into glass fibers are described in Japanese Laid-open Patent Publication (Kokai) No. H1-221433 and Japanese Laid-open Patent Publication (Kokai) No. H8-120573.
Glass fibers used as the core have properties such as having a high tensile strength, having a high modulus and hence little temperature dependence, exhibiting almost elastic deformation upon repeated stretching, and having good dimensional stability to moisture and heat. These properties are particularly desirable for a rubber-reinforcing cord. On the other hand, one of the serious drawbacks of glass fibers is being extremely weak to friction between filaments, resulting in the flexural fatigue resistance, which is an important property required of a rubber-reinforcing cord, being poor. Moreover, another drawback is that adhesiveness to rubber is poor. Consequently, in the case of using glass fibers in a rubber-reinforcing cord, to improve the adhesiveness to the matrix rubber, and to improve the flexural fatigue resistance, it is essential to form a rubber coating.
On the other hand, with a rubber-reinforcing cord in which organic fibers are used as the core, adhesiveness to the matrix rubber can be sufficiently secured through the fiber treatment agent only penetrating in two or three layers from the outermost layer of filaments (where a “filament” is the smallest fiber unit). In the case that the fiber treatment agent penetrates in as far as deep layers, the flexural fatigue resistance may conversely drop, and hence the attachment rate of the fiber treatment agent in the rubber-reinforcing cord is often adjusted to be not more than 10 wt % in terms of solids.
However, with a rubber-reinforcing cord in which glass fibers are used as the core, to prevent abrasion between filaments, it is necessary to make the fiber treatment agent penetrate in as far as the innermost layer of filaments, and hence the attachment rate of the rubber coating (the attachment rate in terms of solids after drying and curing) is necessarily increased to 15 to 25 wt %. A rubber-reinforcing cord in which glass fibers are used as the core is markedly different to a rubber-reinforcing cord in which organic fibers are used as the core in this respect. The properties of a rubber-reinforcing cord in which glass fibers are used as the core are thus greatly affected by the properties of the fiber treatment agent used in treating the core.
With regard to rubber-reinforcing cords in which glass fibers are used as the core, and a fiber treatment agent containing a nitrile group-containing copolymer latex is used as the fiber treatment agent, the present inventors carried out assiduous studies with an aim of improving various properties, and as a result, focusing on the type of the latex in the fiber treatment agent, in particular functional groups affecting the reactivity of the latex, discovered that the oil resistance of a rubber product can be markedly improved by specifying the type and content of the latex.
The present invention was accomplished based on this discovery. It is an object of the present invention to provide a rubber-reinforcing glass fiber treatment agent capable of improving the properties, in particular the oil resistance, of a rubber product, a rubber-reinforcing cord using the fiber treatment agent, and a rubber product having high oil resistance.